While numerous case-hardening processes have been developed over the years for obtaining an increased level of carbon in surface regions of metallic workpieces, especially steel workpieces, increasing interest has centered, in recent years, upon gas carburization and gas carbonitriding, processes in which the steel workpieces are brought to an elevated temperature generally in excess of 900.degree. C. and contacted with a gas mixture which can include a carbon-containing compound and, if desired, a nitrogen-containing compound. With carbonitriding, nitrogen from ammonia, for example, can diffuse into surface regions of the workpiece together with carbon to increase the carbon level and, if desired, the nitrogen level.
The workpiece can form part of a charge introduced into an annealing oven or furnace which is provided with the controlled atmosphere and is brought to the desired reaction temperature, both the atmosphere and the temperature being maintained during the course of the heat treatment.
It has been found to be difficult with gas carburization processes developed to date to obtain reproducible carburization results with variations in the metal alloys which may be provided as the workpieces, differences in contours thereof and differences in the interaction of the workpieces with the gas. As a result, if all other parameters of operation of the annealing furnace are held constant and only the charge is changed, one finds differences in the carbon penetration among workpieces of the charge as well as differences in such penetration between successive charges although the operating conditions were ostensibly identical. It has already been proposed to form the carburization atmosphere from a three-component gas mixture including a carbon carrier, a nitrogen carrier, a hydrogen carrier and an oxygen donor. Thus, for example, ammonia can form the hydrogen and nitrogen carrier, a hydrocarbon can form the carbon and hydrogen carriers, etc. An oxygen donor can also be provided (see German patent documents DE-OS No. 24 50 879 and DE-OS No. 28 18 558).
The carbon carriers suggested are hydrocarbons and preferably the paraffin or paraffinic hydrocarbons, methane, ethane, propane, butane or natural gas with about 57.5 to 38 gram-atoms of carbon.
Suitable oxygen donors include oxygen, air, carbon dioxide, carbon monoxide and steam or mixtures thereof.
The gas components were introduced separately or premixed before being fed to the heat treatment chamber and the measurement and control of the gas atmosphere was effected by dewpoint, measurements, infrared detection of carbon dioxide or oxygen level measurements in the furnace atmosphere.
It has also been suggested to feed methane as a carbon-containing gas to the atmosphere in stages separated by two or three pauses, i.e. phases in which no carbon carrier is introduced.
During the carburization intervals, the carbon potential of the oven atmosphere is sharply raised and soot or carbon black precipitation can occur upon the workpiece. In the next phase, when no carbon carrier is introduced, this precipitated elemental carbon can participate in case hardening. During these pauses in the introduction of the carbon carrier, air is introduced into the chamber and the carbon potential is reduced to zero.
This method has been found to result in edge oxidation zones which are unavoidable. However, without introduction of the oxygen and the consequent elimination of excess precipitated carbon, the process is inefficient and, if one avoids precipitation of carbon, the carburization time is inordinately prolonged.